In the production of sheet materials such as paper, it is necessary to control certain properties of the sheet material. Properties such as basis weight, moisture and caliper will vary along the machine direction which is the path in which the sheet material is moved during production, and these properties will also vary in the cross machine direction which is perpendicular to the machine direction.
In order to control the paper parameters, the sheet material being manufactured must be accurately measured and data concerning the measured parameters used to alter the process to maintain the parameters within desired limits. Collection of samples from scanning measurement systems which traverse the sheet perpendicular to its travel result in the establishment of a high resolution profile. These high resolution profiles are transformed by filtering into a control profile that is used to control actuators that adjust the parameters of the sheet material in a feedback loop.
For example, in order to control sheet thickness in the cross machine direction, calendering machines are used comprising a series of rolls arranged in parallel, one above the other, in a stack. The sheet material is trained through the stack to pass through the nip areas between adjacent rolls. Calender profile actuators are used to adjust the thickness parameter of the sheet material. The profile actuators comprise a plurality of devices that operate to heat or cool the rolls differentially along their length. Each heating or cooling device controls a zone or "slice" of a roll. For example, when a slice of the roll is heated, the diameter of the roll increases and the caliper of the sheet material in that slice is decreased. The caliper of the sheet material downstream of the rolls is monitored by a scanning sensor that collects a plurality of datapoints in the cross machine direction. These datapoints define a high resolution or mini-slice profile of the thickness of the paper sheet. The profile is provided to a profile analyser as a signal indicative of the caliper thickness. The signal is transformed by the profile analyser into a low resolution control profile. The control profile is divided into a plurality of control slices, each control slice being used to adjust a particular actuator to correct for any variation in caliper thickness from a desired profile across the sheet material. Each control slice in the low resolution profile is derived from plurality of mini-slices in the high resolution profile. In other words, each datapoint in the control profile is derived from a plurality of datapoints in the high resolution profile.
Other sheet parameters are controlled in the same general manner. A high resolution profile of a parameter to be controlled is acquired by sensing equipment. The high resolution profile is then transformed into a low resolution control profile that provides control signals to actuators for adjusting the parameter being monitored.
In any control system that works along the foregoing lines, it is important that when the high resolution profile is transformed into the low resolution control profile the datapoints or signal be filtered to prevent aliasing in the control profile. The high resolution profile contains many different frequency components and when the high resolution profile is transformed to a low resolution profile that is aligned to the physical actuator dimensions, the higher frequency components can lead to a distortion of the control profile. Therefore, aliasing is the creation of sampling induced fictitious components that are added to signal content. As a result, control actions can be made on "phantom" variation which does not exist in the original high resolution profile.